Vertebrates possess the ability to mount an immune response as a defense against pathogens from the environment as well as against aberrant cells, such as tumor cells, which develop internally. The immune response is the result of complex interactions between a variety of cells and factors, but generally comprises two main facets. One is a cellular component, in which specialized cells directly attack an offending agent (bearing an antigen) while the other is a humoral component, in which antibody molecules bind specifically to the antigen and aid in its elimination. Acting in concert, the individual elements are quite effective in limiting the initial onslaught of invading pathogens and eliminating them from the host.
The primary cells involved in providing an immune response are lymphocytes, which generally comprise two principal classes. The first of these, designated B cells or B lymphocytes, are typically generated in bone marrow and are, among other duties, responsible for producing and secreting antibodies. B cell antibody products tend to react directly with foreign antigens and neutralize them or activate other components of the immune systems that then eliminate them. In particular, opsonizing antibodies bind to extracellular foreign agents thereby rendering them susceptible to phagocytosis and subsequent intracellular killing. On the other hand, T cells or T lymphocytes, which generally develop or mature in the thymus, are responsible for mediating the cellular immune response. These cells do not recognize whole antigens but, instead, respond to short peptide fragments thereof bound to specialized proteins that appear on the surface of the surface of a target cell as well as an antigen presenting cell. More particularly, it appears that proteins produced within the cell, or taken up by the cell from extracellular milieu, are continually degraded to peptides by normal metabolic pathways. The resulting short fragments associate with intracellular major histocompatibility complex (MHC) molecules and the MHC-peptide complexes are transported to the surface of the cell for recognition by T cells. Thus, the cellular immune system is constantly monitoring a full spectrum of proteins produced or ingested by the cells and is posed to eliminate any cells presenting foreign antigens or tumor antigens; i.e. virus infected cells or cancer cells.
The structure of immunoglobulin G (IgG) is that of a tetrameric protein complex comprising two identical heavy (H) chains and two identical immunoglobulin light (L) chains. These chains are joined together by disulfide bonds to form the Y-shaped antibody complex. In solution however, the molecule takes on a more globular shape and readily bind to foreign antigens present in biological fluids. Amino acid sequence analysis of immunoglobulins has led to the definition of specific regions with various functional activities within the chains. Each light chain and each heavy chain has a variable region (VL and VH respectively) defined within the first 110 amino terminal residues. Three dimensional pairing of the VL and VH regions constitute the antigen-recognition portion or “antigen combining site” (“ACS”) of immunoglobulin molecule. Because of the tetrameric nature of immunoglobulins, there are two identical antigen combining sites per molecule. The variable domains of these chains are highly heterogeneous in sequence and provide the diversity for antigen combining sites to be highly specific for a large variety of antigenic structures. The heterogeneity of the variable domains is not evenly distributed throughout the variable regions, but is located in three segments, called complementarity determining regions (“CDRs”) designated CDR 1, CDR 2 and CDR 3. For further information regarding these structures see Watson et al., 1987, Molecular Biology of the Gene, Fourth Edition, Benjamin/Cummings Publishing Co., Inc. Menlo Park, Calif. incorporated herein by reference.
Each of the heavy chains also includes a constant region defining a particular isotype and assigns the immunoglobulin to one of the immunoglobulin classes and subclasses. The constant region contains units called domains (i.e. CH1, CH2, etc.) that do not vary significantly among antibodies of a single class. The constant region does not participate in antigen binding, but can be associated with a number of biological activities known as “effector functions”, such as binding to Fc receptors on cell surfaces as well as binding to complement proteins. Antigen presenting cells such as dendritic cells and macrophages are, among other features, generally distinguished by the presence of an Fc receptor. Consequently, if an antibody is bound to a pathogen, it can then link to a phagocyte via the Fc portion. This allows the pathogen to be ingested and destroyed by the phagocyte, a process known as opsonization. Moreover, as will be discussed in more detail below, various pathogenic antigens may be processed and displayed by the APC to further stimulate an immune response.
Unlike the heavy chains, the light chains have a single constant domain (CL). A light chain pairs with a heavy chain through a disulfide bond which attaches heavy constant region CH1 to CL. In addition, the heavy chains have a hinge region separating constant regions CH1 and CH2 from the remainder of the molecule. It is this hinge region that is largely responsible for the flexibility of the tetramer. The two heavy chains of the molecule pair together through disulfide bonds at the junction between the hinge region and CH2.
In order to provide such an extensive repertoire, immunoglobulin genes have evolved so as permit the production of vast numbers of different immunoglobulin proteins from a finite number of genes i.e. inherent polymorphism. Due to inherent polymorphism, mammals are able to produce antibodies to a seemingly infinite variety of antigens. For a review of immunoglobulin genetics and protein structure see Lewin, “Genes III”, John Wiley and Sons, N.Y. (1987) and Benjamini and Leskowitz, 1988, Immunology, Alan R. Liss, Inc., New York, which is incorporated herein by reference.
In the past few years antibodies have become extremely important in diagnostic and therapeutic applications due to their diversity and specificity. Increasingly, molecular biology techniques have been used to expand the variety and availability of antibodies for scientific applications. For instance, a single antibody producing B cell can be immortalized by fusion with a tumor cell and expanded to provide an in vitro source of antibodies of a single specificity known as a “monoclonal antibody” (mAb). Such an immortal B cell line is termed a “hybridoma.”
Until recently, the source of most mAb has been murine (mouse) hybridomas cultured in vitro. That is, a mouse was typically injected with a selected antigen or immunogen. Subsequently, the animal was sacrificed and cells removed from its spleen were fused with immortal myeloma cells. Although they have been used extensively in diagnostic procedures, murine mAb are not well suited for therapeutic applications in most mammals including humans. In part, this is due to the fact that murine antibodies are recognized as foreign by other mammalian species and elicit an immune response that may itself cause illness.
To overcome at least some of the problems of immune responses generated by foreign mAb and the lack of suitable human mAb, genetic engineering has been used to construct humanized chimeric immunoglobulin molecules which contain the antigen binding complementarity determining regions of the murine antibodies but in which the remainder of the molecule is composed of human antibody sequences which are not recognized as foreign. Such antibodies have been used to treat tumors as the mouse variable region recognizes the tumor antigen and the humanized portion of the molecule is able to mediate an immune response without being rapidly eliminated by the body. See, for example, Jones et al., Nature, 321:522-525 (1986), which is incorporated herein by reference.
Other uses of such antibodies are detailed in PCT Publication No. WO 94/14847, which is also incorporated herein by reference. In these cases epitopes of foreign antigens such as viral or bacterial epitopes are grafted onto the hypervariable region of an immunoglobulin to induce a response. That is, the engineered antibodies are used as a vaccine to provoke an immune response and confer long-term immunogenic memory thereby allowing the subject to fight off subsequent infections.
These and more traditional vaccines are effective in that they stimulate both prongs of the immune system. Despite the intricacies associated with the humoral component of the immune response, it would not, in and of itself, be capable of effectively protecting an animal from the myriad pathogenic assaults to which it is subject each day. Rather, it is only the presence of a highly evolved cellular response that allows higher organisms to survive and proliferate.
As indicated above, T lymphocytes or T cells, which arise from precursors in the bone marrow, are central players in the immune response against invading viruses and other microbes. The progenitor stem cells migrate to the thymus where, as so-called thymocytes, they become specialized. In particular, they begin to display the receptor molecules that later enable mature T cells to detect infection To be beneficial, T cells must be able to attach through their receptors to antigens (protein markers signaling an invader's presence). At the same time, they should be blind to substances made by the body as self-reactive T cells can destroy normal tissues. Typically, only those thymocytes that make useful receptors will mature fully and enter the bloodstream to patrol the body. Others that would be ineffectual or would attack the body's own tissue are, in healthy individuals, eliminated through apoptosis prior to leaving the thymus.
Mature T cells that finally enter the circulation, either as cytolytic T lymphocytes or T helper cells, remain at rest unless they encounter antigens that their receptors can recognize. Upon encountering the specific antigens for which the lymphocytes have affinity, they proliferate and perform effector functions, the result of which is elimination of the foreign antigens.
T cells have been classified into several subpopulations based on the different tasks they perform. These subpopulations include helper T cells (Th), which are required for promoting or enhancing T and B cell responses; cytotoxic (or cytolytic) T lymphocytes (CTL), which directly kill their target cells by cell lysis; and suppressor or regulatory T cells (Ts or Tr) which down-regulate the immune response. In every case T cells recognize antigens, but only when presented on the surface of a cell by a specialized protein complex attached to the surface of antigen presenting cells. More particularly, T cells use a specific receptor, termed the T cell antigen receptor (TCR), which is a transmembrane protein complex capable of recognizing an antigen in association with the group of proteins collectively termed the major histocompatibility complex (MHC). Thousands of identical TCR's are expressed on each cell. The TCR is related, both in function and structure, to the surface antibody (non-secreted) which B cells use as their antigen receptors. Further, different subpopulations of T cells also express a variety of cell surface proteins, some of which are termed “marker proteins” because they are characteristic of particular subpopulations. For example, most Th cells express the cell surface CD4 protein, whereas most CTL cells express the cell surface CD8 protein and Tr cells expressed CD25 and CD4 molecules. These surface proteins are important in the initiation and maintenance of immune responses that depend on the recognition of, and interactions between, particular proteins or protein complexes on the surface of APCs.
For some time it has been known that the major histocompatibility complex or MHC actually comprises a series of glycosylated proteins comprising distinct quaternary structures. Generally, the structures are of two types: class I MHC which displays peptides from proteins made inside the cell (such as self-proteins or proteins produced subsequent to viral replication), and class II MHC, which generally displays peptides from proteins that have entered the cell from the outside (soluble antigens such as bacterial toxins). Recognition of various antigens is assured by inherited polymorphism that continuously provides a diverse pool of MHC molecules capable of binding any pathogenic peptides that may arise. Essentially, all nucleated cells produce and express class I MHC, which may exhibit naturally occurring peptides, tumor associated peptides or peptides produced by a viral invader. Conversely, some other nucleated cells and among them specialized lymphoid cells, those generally known as antigen presenting cells, produce and express class II MHC proteins. Regardless of the cell type, both classes of MHC carry peptides to the cell surface and present them to resting T lymphocytes. Ordinarily, Th cells recognize class II MHC-antigen complexes while CTL's tend to recognize class I MHC-antigen complexes, although cross-presentation of antigens also occurred
When a resting T cell bearing the appropriate TCR encounters the APC displaying the peptide on its surface, the TCR binds to the peptide-MHC complex. More particularly, hundreds of TCR's bind to numerous peptide-MHC complexes. When enough TCRs are contacted the cumulative effect activates the T cell. Receptors on T cells that are responsible for the specific recognition of, and response to, the MHC-antigen complex are composed of a complex of several integral plasma membrane proteins. As with the MHC complex previously discussed, a diverse pool of TCR's is assured by inherent polymorphism leading to somatic rearrangement. It should be emphasized that, while the pool of TCR's may be diverse, each individual T cell only expresses a single specific TCR. However, each T cell typically exhibits thousands of copies of this receptor, specific for only one peptide, on the surface of each cell. In addition, several other types of membrane associated proteins are involved with T cell binding and activation.
Activation of the T cell entails the generation of a series of chemical signals (primarily cytokines) that result in the cell taking direct action or stimulating other cells of the immune system to act. In the case of class I MHC-antigen activation, CTL's proliferate and act to destroy infected cells presenting the same antigen. Killing an infected cell deprives a virus of life support and makes it accessible to antibodies, which finally eliminate it. In contrast, activation of Th cells by class II MHC-antigen complexes does not destroy the antigen presenting cell (which is part of the host's defense system) but rather stimulates the Th cell to proliferate and generate signals (again primarily cytokines) that affect various cells. Among other consequences, the signaling leads to B cell stimulation, macrophage activation, CTL differentiation and promotion of inflammation. This concerted response is relatively specific and is directed to foreign elements bearing the peptide presented by the class II MHC system.
Constant surveillance of epitopes throughout those structures in the body accessible to the immune system provides a very effective means for recognizing and maintaining “self” and destroying epitopes and their carriers that invade the body or arise pathologically. When operating properly the immune response is surprisingly effective at eliminating microscopic pathogens and neoplastic (tumor) cells that are believed to arise continuously in the body and for the most part are eliminated by the immune system before becoming detectable. Certain regions of the body, such as the brain, eye, and testis, are protected from immune surveillance, these sites are referred to as immune privileged. In general, the complicated mechanisms for self-recognition are very efficient and allow a strong response to be directed exclusively at foreign antigens. Unfortunately, the immune system occasionally malfunctions and turns against the cells of the host provoking an autoimmune response. Typically, autoimmunity is held to occur when the antigen receptors on immune cells recognize specific antigens on healthy cells and cause the cells bearing those particular substances to die. In many cases, autoimmune reactions are self-limited in that they disappear when the antigens that set them off are cleared away. However, in some instances the autoreactive lymphocytes survive longer than they should and continue to induce apoptosis or otherwise eliminate normal cells.
Current data indicates that immune protection against all cancers requires the generation of a potent cellular immune responses against a unique tumor antigen expressed by the malignant cell. As a consequence, successful immune protection first requires a unique antigen expressed in the tumor cells (tumor-specific antigen) and second, induction of a potent T cell immune response targeted to the tumor antigen.
Several tumor-associated antigens are currently known, and have been used in pre-clinical and clinical studies for generating vaccines. For example, PSMA, PAP and PSA are antigens expressed in prostate tumor cells. Her2/neu and MUC1 are antigens expressed by breast cancer cells and other carcinomas, including carcinomas of the lung, ovary, colon, and pancreas. MAGEs and MART-1 are melanoma tumor cell-associated antigens, and CEA is an antigen associated with pancreas or colorectal cancer. Other tissue and/or tumor specific antigens also have been described. However, while all of these antigens are expressed in tumor cells in the normal or aberrant forms, they are also expressed in a variety of normal cells, and thus cannot be used for prophylactic vaccination. In other words, these tumor-associated antigens are still recognized by immune cells as self-molecules and so no true activation of the immune system occurs. This presents at least two obstacles for targeting these tumor-associated molecules as the basis for a vaccine. The first obstacle is the unresponsiveness (tolerance) of the immune system to self-molecules, which restricts its ability to generate potent cellular immune responses. The second is that any potent cellular immune response generated should not be directed toward normal cells that express the target antigen. This is the reason that all the tumor-associated antigens discussed above are suggested for use only as targets for therapeutic vaccinations.
A new protein has been recently described that is able to overcome the problems associated with the known tumor-associated antigens. Brother of Regulator of Imprinted Sites (BORIS) was first described as a DNA-binding protein found in testis. This protein shares 11 zinc-finger (ZF) domains with CCCTC-binding factor (CTCF) that is a multivalent 11-zinc finger nuclear factor. CTCF is a conserved, ubiquitous and highly versatile factor involved in various aspects of gene regulation and which forms methylation-sensitive insulators that regulate X chromosome inactivation and expression of imprinted genes. BORIS differs from CTCF, however, at the N and C termini and is expressed in a mutually exclusive manner with CTCF during male germ cell development. BORIS expression is restricted to the testis and then only within a select cell subpopulation of spermatocytes that are involved with the re-setting of methylation marks during male germ cell development. This testis cell subpopulation is also the only normal cell type known that does not express CTCF. Because inhibition of CTCF expression in cultured cells leads to apoptosis, it is reasonable to assume that BORIS is activated to maintain some of the vital CTCF functions in testis cells (Loukinov et al. (2002) Proc. Natl. Acad. Sci. 99(10):6806-6811; which is incorporated herein by reference).
More recently, it was demonstrated that while CTCF overexpression also blocks cell proliferation, expression of BORIS in normally BORIS-negative cells promotes cell growth that can lead to transformation (Klenova et al. (2002) Cancer Biol. 12:399-414; which is incorporated herein by reference). Human BORIS maps to the 20q13 region, which is well known for frequent gains and/or amplifications observed in many of the same types of tumors that also often show loss of heterozygosity (LOH) at the paralogous locus on 16q22 where CTCF resides. These regions are associated with “hot-spots” associated with breast, prostate, ovarian, gastric, liver, endometrial, glioma, colon and esophageal cancer as well as Wilms tumors. Importantly, abnormal activation of BORIS expression appears to be found in a significant proportion of a wide variety of neoplasms. Using Northern blots or RT-PCR, Klenova et al. (2002) analyzed BORIS mRNA levels in over 200 cancer cell lines representing most of the major forms of human tumors and detected transcripts in more than one half of the cell lines tested. Subsequent analysis of primary cancers, for breast cancer samples, confirmed the results obtained with the cell lines.